Using independent frequencies probably has no massive negative aspect.
Identical frequencies, if they really are identical, may lead to switching transients from one converter getting into another at a critical part in the switching cycle and affecting when and how it switches. At the likely levels involved in cross feeding I would expect this usually not to be a fatal problem - just MAY degrade accuracy of output if switching point is slightly affected by other signals.
Such spurious input would usually have effect only where they affected the switching decision voltage when it was very near to the switching threshold as at other points in the cycle the voltages will be large enough that noise will not affect them. eg if switching point occurs when divided down output is fed to Vref pin and = Vref = say 0.8V, then if Vin is 0.799 V, noise on the sense line that is coupled onto Vsense and that results in a + 0.001V change may make it switch early. But if Vsense is at 0.700 Volts you'd need +0.1V noise to trigger switching
Higher frequency Noise on Vout has a fairly free ride on to Vsense as there is usually a cap across the reference divider from Vsense to Vout. This vastly improves response time to transients and removal can lead to a converter which is fatally trying to chase its own tail. Ask me how I know :-).
If N converters are asynchronous wrt frequency etc then glitches will occur pseudo randomly throughout the cycle and hopefully have little overall effect.